Unlike conventional illuminants, such as incandescent light bulbs, light emitting diodes are usually supplied with a constant operating current rather than a constant operating voltage. Therefore controllable current source circuits are usually employed for driving LED devices, and switching converters are used to keep power losses due to the current conversion low. Various integrated LED controller circuits including, e.g., a buck converter for current conversion are readily available (e.g., integrated LED controller ILD4120 from Infineon).
As a matter of operating principle the LED current (i.e., the load current supplied to the LED device) will always exhibit a ripple when using a current supply including a switching converter such as a buck converter. In order to enable a (controllable) constant current supply the LED current is usually measured (e.g., using a sense resistor coupled in series to the LED device), and a current signal is fed back to the controller circuit. The (measured) current feedback signal may then be compared with respective thresholds and a switching operation may be triggered when the feedback signal reaches a threshold value. Thus, the threshold values determine the size of the current ripple which may be designed to amount, for example, ±15 percent around the average LED current.
Various losses occur in such LED controller circuits such as losses due to a finite (non-zero) on-resistance of the power transistor switching the load current, losses due to the sense resistor, and losses dissipated in the required free-wheeling diode (e.g., a Schottky diode). The actual current ripple depends on the propagation delay between the time instant when the current feedback signal reaches a threshold value and the time instant when the corresponding switching operation is accomplished by the respective power transistor. Generally, the influence of the propagation delay on the current ripple can be compensated for by appropriately setting the mentioned threshold values. However, when using such an approach the achieved compensation is only valid for one specific set-up (i.e., a specific number of LEDs, a specific inductor used in the buck converter, a specific operating voltage, etc.). For example, the threshold values may be designed such that a desired ripple current of ±15 percent around the average LED current is achieved for an operating voltage of 12V, an inductor of 68 μH, and a LED device including three white LEDs connected in series. If one of these parameters (i.e., number of LEDs, operating voltage, inductance, etc.) changes, the actual current ripple will deviate from its desired value. A smaller current ripple entails a higher switching frequency and thus higher switching losses. A higher current ripple may be undesired for different reasons (e.g., a maximum current ripple may be specified by the customer).
In view of the above there is a need for a LED controller including an improved current ripple control.